Direct current (DC) power systems are becoming increasingly common. For example, DC power systems are being used in solar panel farms, rail systems, and telecommunication systems. To safely and efficiently monitor the operation of a DC power system it is important to sense the DC current that it develops. In some cases, ground-fault current detection is also needed to protect equipment and personnel. The requirement for detecting DC current levels involved in ground faults are normally in the low, milliampere range.
A magnetometer can be used to measure DC current flow without directly contacting the conductor through which it is flowing by sensing the magnetic field around the conductor generated by the current flow. A magnetometer relies upon Ampere's law, which holds that the integral of the magnetic field B tangential to an electrically conductive closed path is proportional to the net current enclosed by the area surrounding the path. Hence, sensing the magnetic field of a circular closed path that encloses a conductor will provide the necessary information to calculate the net current flowing through the conductor.
Prior art magnetometers typically use either Hall Effect sensors or fluxgate sensors. A Hall Effect sensor is a direct magnetic field strength sensor that, based upon current technology, has an effective sensitivity down to approximately 0.5 amperes (A), and thus is not suited for sensing current in the milliampere (mA) range. A fluxgate sensor is based on Faraday's law of induction to sense the flux change (−dφ/dt) disturbance by a “sense current”, e.g. a current flowing through a conductor. One type of fluxgate sensor uses a single core, which has the disadvantage of being sensitive to earth magnetic field bias if the windings are distributed unevenly (resulting in an inconsistent bias being applied to the circuit when the core is oriented in different attitudes) or capacitive coupling between the windings, if they are distributed evenly, which can be injected into the sensing signal. While more complicated and expensive, a dual core configuration can mitigate the aforementioned shortcomings of the single core configuration.
U.S. Pat. No. 7,378,843 describes a magnetic field measuring device, equipped with a fluxgate magnetometer, having at least one magnetic core and a plurality of windings, and configured to deliver at least one output signal and pulse generating means for emitting at least one excitation signal at the input of the magnetic sensor, in the form of a succession of excitation pulses. U.S. Pat. No. 7,355,397 describes a fluxgate magnetometer drive circuit includes a fluxgate inductor that is driven through magnetic saturation by altering voltage pulses. U.S. Pat. No. 4,929,899 describes a fluxgate magnetometer having wide temperature range accuracy that aligns a permeable magnetic tape core relative to a secondary winding and increasing a fluxgate magnetometer primary drive current above what is normally considered saturation current to reduce sensitivity of the output to temperature and primary drive changes.
Fluxgate technology is advantageous in that it can be used to measure currents down to the mA range. However, prior art fluxgate magnetometers, whether they are single or dual core types, consume considerably more power than Hall Effect magnetometers due to the need to drive the magnetic core(s) into magnetic flux saturation by the continuous application of an alternating current (AC) into the core windings.
These and other limitations of the prior art will become apparent to those of skill in the art upon a reading of the following descriptions and a study of the several figures of the drawing.